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OPA847: [ OPA657 & OPA847 ] - 2 Stage Transimpedance Amplifier Oscillation.

Part Number: OPA847
Other Parts Discussed in Thread: OPA657,

Hello Everyone, how are you? I have an oscillation issue on a photodiode transimpedance amplifier whose  simplified schematic i'm attaching in this thread. The design consist of a high gain / high bandwidth

transimpedance stage using a JFET Input OPA657 (SOT23-5 Package), followed with a voltage gain of 101V/V  made with an OPA847 (SOT23-6 Package). All components used are 0805 SMD technology. The entire gain of the amplifier is intented to be 10Mohms, having a bandwidth of 10MHz.

C1 represents the photodiode capacitance at certain bias voltage, C2 Represents the parasitic capacitance of the SMD 0805 100K resistor (R1).  Each opamp has 3 bypass capacitors on each supply pin (0805/10pF - Ceramic, 1206/100nF Tantalum, 1206/10uF Aluminum). All of them are very close to the pins. I followed all the indications on the application notes of each opamp to make the entire PCB. The implementation was made in a 2 layer, FR4 PCB that has a perfect (unbroken) ground plane.

The circuit oscilates around 10 MHz. Here are the things i tested to figure out what kind of problem i have (without success):

- Changing the resistor values (keeping in mind the gain values on which each amplifier is stable).

- Removing the 10pF ceramic capacitors on each supply pins (on the datasheets, the proposed values of the capacitors are 100nF and 6.8uF).

- Putting a capacitor in parallel on R2 (the OPA847 is stable for gains > 12 V/V, but i tried that anyway).

- I simulated the (extreme) worst case scenario of the circuit putting parasitic capacitances of 1pF on the inverting and noninverting pins of the opamps to see the effects of a bad pcb, the simulations works fine.

- Keeping in mind the last item, i made a stability analisys opening both feedbacks on each stage to see the phase margin. In the firs amplifier i have 39° of PM, and in the second 60°.

- I replaced both opamps.


I opened the circuit between opamps to test each stage. The Transimpedance stage works fine (i tested it with a amplitude-modulated light source at 10 MHz) but the second stage oscillates.

I made this circuit because i have already made a successful previous prototype on which i emulated the photodiode with a capacitor and a voltage source to made a characterization of the circuit. The only difference was on the OPA847 package. On the previous prototype i used the package SO-08.

¿It is only a PCB-Related issue? Is there any other test i can do to figure out what's going on?

Thanks in advance!